It is well known to deposit a coating on an electrically conductive filament using chemical vapour deposition techniques. Typically the electrically conductive filament is passed continuously through a deposition chamber containing an appropriate gas or gases whilst the filament is heated by the passage of an electrical current, and the gas or gases deposit a coating on the hot filament. This is a process of “chemical vapour deposition” or CVD and essentially requires the provision of gas seals around the electrical contacts to the filament at both ends of the deposition chamber.
EP 0 396 333 teaches that silicon carbide may be coated on a tungsten filament which passes through electrodes at the ends of a deposition chamber, the entrance electrode is a pool of mercury and the exit electrode is a mercury/indium amalgam. The pool of mercury and the mercury/indium amalgam both serve the dual function of providing a gas seal around and an electrical contact to the tungsten filament.
U.S. Pat. No. 3,622,369 and U.S. Pat. No. 4,127,659 both describe similar processes for depositing silicon carbide on a filament.
EP 0 396 332 teaches that an exit electrode for a ceramically coated filament should, instead of using mercury, utilise a liquid metal mixture of mercury/indium or mercury/cadmium amalgam or a gallium/indium mixture.
EP 0 450 760 teaches that carbon may be coated on a filament which comprises a tungsten core coated with silicon carbide and is passed through mercury electrodes at the ends of a deposition chamber.
EP 0 598 491 teaches that a layer of titanium carbide can be deposited on a tungsten core as an intermediate layer, an outer layer being of silicon carbide. Again, mercury electrodes are used at the ends of the deposition chamber.
These CVD techniques for producing coated filaments can be applied to different electrically conductive core materials capable of being heated electrically by the direct application of electrical current, or by induction, and to a range of coatings provided by an appropriate selection of reactive gas or gases.